If you do the math, 720 mins = 12 hours, ive never done a cook with the Dog that went over 6 hours.
If I recall correctly if the timer runs out it just goes into manual mode with --:-- blinking on the display, I could be wrong of course.
EDIT: Taken from my dog manual
Q: My SV display is blinking - - : - -
A: This signifies your TC timer set in Menu #1 has expired.